1. Technical Field of the Invention
The present invention relates to an image forming apparatus which prevents occurrence of inconsistencies in density or color of an image.
2. Description of the Related Art
In the field of an image forming apparatus such as a printer, a known optical scanning device is of a type where an optical beam to be caused to enter, e.g., a light-deflecting reflector of a rotary polygon mirror, or the like, and a deflected light beam reflected from the light-deflecting reflector are reflected by a mirror. This optical scanning device is provided with a window which allows an exit of a light beam to a surface to be scanned (hereinafter called a “scan subject surface”) such as an image carrier (a photo conductor).
FIGS. 15A to 15C are explanatory views each showing an example image forming apparatus having such an optical scanning device. FIG. 15A is a diagrammatic front view of the image forming apparatus; FIG. 15B is a side view of the same; and FIG. 15C is a diagrammatic front view showing another configuration of the image forming apparatus. In FIGS. 15A and 15B, reference numeral 5 designates a photo conductor; 6 designates a beam exit window; 16 designates a deflected light beam; 17 designates a scanning line; 20 designates an optical scanning device; 52 designates a duct; 53 designates a fan for sucking out air currents (hereinafter called an “exhaust fan”); 55 designates a cleaner for collecting unwanted toner adhering to the photo conductor 5; 57 designates an inflow side of the air currents circulating around the beam exit window 6; and 58 designates an outflow side by way of which the air currents flow to the outside.
In the electrophotographic process, a space—through which a write beam emitted from the optical scanning device toward the photo conductor passes—is in close proximity to an charging device and a development unit, which are omitted from the illustrations, and the photo conductor 5. There may arise a case where air currents are caused to flow in the space as a measure against ozone or splashes of toner. There is performed processing of activating the exhaust fan 53, such as that shown in FIG. 15A, to thus cause the air currents to flow around the beam exit window 6 by way of the duct 52 and to pass through a filter or the like. Thus, the exhaust fan 53 is disposed in a position downstream of the air currents flowing thereabout and has the function of sucking out the air currents existing around the beam exit window 6.
FIG. 15C shows an example using an intake fan 54 in lieu of the exhaust fan 53. The intake fan 54 is disposed at a position upstream of the air currents flowing around the beam exit window 6 and has the function of drawing the air currents to the neighborhood of the beam exit window 6. The exhaust fan 53 or the intake fan 54 can be arbitrarily selected as a measure against ozone in consideration of the space where components of the image forming apparatus are arranged.
When the image forming apparatus operates, the beam exit window 6 of the optical scanning device 20 is stained by fine powder, such as splashes of toner, with lapse of time, which in turn induces a drop in the power of light arriving at the photo conductor 5. When the exhaust fan 53 or the intake fan 54 is activated, minute dust, such as splashes of toner, adheres to the beam exit window 6 while being carried on the air currents, to thus decrease the quantity of light (power of light) reaching the photo conductor 5. A decrease in the power of light results in an increase in the density of an image, which in turn induces inconsistencies in density.
FIG. 16 is a descriptive view showing a decrease in the quantity of light stemming from stains on the beam exit window 6. FIG. 16(a) corresponds to FIG. 15A. FIG. 16(b) is a characteristic chart showing time-varying changes in stains on the beam exit window 6, wherein the degree of stain is great in the direction of CH but low in the direction of CL. Reference numeral 18 designates the height of an image. FIG. 16(c) is a characteristic chart showing time-varying changes in the quantity of light induced by stains on the beam exit window 6, wherein the quantity of light is high in the direction of VH but low in the direction of VL. Broken line Vx designates the distribution of the initial quantity of light, and solid line Vy designates the distribution of light quantity acquired with lapse of time. As shown in FIG. 16, the degree of stain is large downstream of the air currents circulating through the neighborhood of the beam exit window 6, and the quantity of light is decreased. Next, the reason for this will be described.
In FIG. 16(a), a fine powder, such as the toner having passed through a cleaner of the photo conductor 5, intrudes into the duct 52 provided as a measure against ozone while adhering to the photo conductor 5, and is conveyed downstream while being carried on the air currents in the duct 52. The fine powder in the duct 52 adheres to the beam exit window 6 to thus stain the same, which in turn results in a drop in the quantity of light. Especially, the density of contaminants is increased downstream of the air currents, so that stains on the beam exit window 6 are likely to become worse. As shown in FIG. 16(c), comparison of the air currents blowing over the beam exit window 6 at a downstream position Ab with the air currents blowing over the beam exit window 6 at an upstream position Aa shows that the drop in the quantity of light caused with lapse of time in the position Ab becomes greater.
When the air currents, such as that mentioned previously, has arisen in the axial direction of the photo conductor, stains on the beam exit window of the optical scanning device become noticeable especially at a downstream position of the air currents. There may arise a case where fluctuations occur in the quantity of writing light in the axial direction of the photo conductor, thereby inducing inconsistencies in the density of a printed image. However, the related-art image forming apparatus has not been provided with a measure against a drop in the quantity of light attributable to stains on the beam exit window such as that mentioned previously. Therefore, the related-art image forming apparatus encounters a problem of a failure to prevent deterioration of image quality.
Furthermore, in the field of an image forming apparatus such as a printer, there is an another case where a non-contact charging device is used as charging means. FIGS. 17A to 17C are explanatory views each showing an example of such an charging device. FIG. 17A is a diagrammatic front view of the charging device; FIG. 17B is a side view of the same; and FIG. 17C is a diagrammatic front view showing the charging device of another configuration. In FIGS. 17A and 17B, reference numeral 5 designates a photoconductor; 51 designates an charging device; 52 designates a duct; 53 designates an exhaust fan; 55 designates a cleaner for collecting the unwanted toner adhering to the photoconductor 5; 57 designates an inflow side of air currents circulating around the charging device 51; 58 designates an outflow side of the air currents; and 20 designates an optical scanning device for scanning the photoconductor through use of a deflected light beam 16.
Since a high voltage is applied to the charging device 51, there is a problem of ozone developing around the charging device. The exhaust fan 53, such as that shown in FIG. 17A, is activated as an example of a measure against ozone, to thus cause air to flow around the charging device 51 by way of the duct 52 and pass through a filter, or the like. As mentioned above, the exhaust fan 53 is disposed downstream of the air currents flowing around the charging device, and has the function of sucking the air currents present around the charging device.
FIG. 17C shows an example where an intake fan 54 is used in lieu of the exhaust fan 53. The intake fan 54 is disposed in a position upstream of the air currents flowing around the charging device and has the function of blowing the air current into the neighborhood of the charging device. The exhaust fan 53 or the intake fan 54 can be arbitrarily selected as a measure against ozone in consideration of the space where components of the image forming apparatus are arranged. When the exhaust fan 53 or the intake fan 54 is activated, minute dust, such as splashes of toner, adheres to the charging device 51 while being carried on the air currents, to thus deteriorate charging performance. A deterioration in charging performance results in an increase in the density of an image, which in turn induces inconsistencies in density.
FIG. 18 is a descriptive view showing a decrease in charging performance derived from such contamination of the charging device. FIG. 18(a) corresponds to FIG. 18(a). FIG. 18(b) is a characteristic chart showing time-varying changes caused by contamination of the charging device, wherein the degree of contamination is great in the direction of CH, and the degree of contamination is low in the direction of CL. Reference numeral 18 designates the height of an image. FIG. 18(c) is a characteristic chart showing time-varying changes in charging performance, wherein charging performance is high in the direction of DX, and charging performance is low in the direction of DL. Broken line Dx designates initial charging performance, and solid line Dy designates charging performance achieved with lapse of time. As shown in FIG. 18, the degree of contamination is high at downstream of the air currents circulating around the neighborhood of the charging device, and charging performance is deteriorated. Next, the reason for this will be described.
In FIG. 18(a), a fine powder 59, such as the toner having passed through a cleaner of the photoconductor 5, intrudes into the duct 52 provided as a measure against ozone while adhering to the photoconductor 5, and is conveyed downstream while being carried on the air current in the duct 52. When the fine powder in the duct 52 adheres to the charging device 51 to thus stain the same, charging performance drops. Especially, the density of contaminants is increased at the downstream end of the air currents, so that stains are likely to accumulate on the charging device 51. As shown in FIG. 18(c), comparison of the air currents blowing to the charging device 51 at a downstream position Aa with the air currents blowing to the charging device 51 at an upstream position Ab shows that time-varying drop in charging capability at the position Aa becomes greater.
The image forming apparatus of this type uses an optical scanning device having a rotary polygon mirror as an exposure device. JP-A-5-236214 (hereinafter referred to as JPA′214) describes an image forming apparatus equipped with such an optical scanning device. Specifically, JPA′214 describes a charging device and an optical scanning device having a rotary polygon mirror, such as those described by reference to FIGS. 17A and 18.
As described by reference to FIG. 18, in the image forming apparatus using a non-contact charging device, air currents are caused to circulate around the neighborhood of the charging device as a measure against ozone. However, stains on the charging device become irregular because of fine powder, such as toner mixedly present in the air currents. In an example shown in FIG. 18(b), the degree of contamination achieved at the downstream end of the air currents becomes greater. Thus, as a result of the degree of stains on the charging device becoming irregular, inconsistencies arise in the density of a printed image. In short, stains on the charging device result in a drop in charging performance; the density of a print tends to easily become higher; and charging performance achieved at the downstream end of the air currents is deteriorated. The image forming apparatus described in JPA′214 encounters a problem of the ability to address deterioration of image quality induced by a drop in charging performance.
Furthermore, in the field of an image forming apparatus such as a printer, a photoconductor having a photosensitive layer being formed on the surface thereof is used as a photoconductor. FIG. 19 is an explanatory view of an example where such a photosensitive layer is formed. As shown in FIG. 19, the photosensitive layer of a photoconductor 52 is formed by means of dipping (a dip-coating method) the photoconductor into a coating fluid contained in a container 56. In FIG. 19, the photoconductor 52 is gripped by means of a jig 51; dipped in a coating fluid as indicated by arrow X; and pulled, whereby a photosensitive layer 54 is formed on the surface of the photoconductor 52. In relation to the thus-formed photosensitive layer 54, a deviation of 1 to 2 μm arises between the film thickness of the photosensitive layer formed over an upper portion 53 of the photoconductor and the film thickness of the photosensitive layer formed over a lower portion 55 of the photoconductor, under the influence of an electric-charge generation layer and an electric-charge transport layer.
For instance, the photosensitive layer formed over the upper portion 53 of the photoconductor is smaller in film thickness than the photosensitive layer formed over the lower portion 55 of the photoconductor by an amount of 1 to 2 μm. The potential of the electric charges on the surface of the photoconductor is inversely proportional to the electrostatic capacitance of the photosensitive layer. Specifically, the charging potential of the surface of the photoconductor is proportional to the film thickness of the photosensitive layer. When the film thickness is large, the potential increases. Consequently, the distribution of sensitivity achieved during exposure varies in the axial direction of the photoconductor (in the longitudinal direction; namely, the main scanning direction).
FIG. 20 is a descriptive view showing a relationship between the distribution of an axial film thickness of the photoconductor 5 and the distribution of sensitivity of the same. FIG. 20(c) is a diagrammatic perspective view of the photoconductor 5; FIG. 20(d) is a front view of the same; and FIG. 20(e) is a side view of the same. FIG. 20(a) is a characteristic chart showing the distribution of film thickness, wherein the horizontal axis corresponds to axial positions of the photoconductor, and the vertical axis corresponds to film thickness. Reference symbol SH designates a direction in which a film thickness is high, and SL designates a direction in which a film thickness is low. FIG. 20(b) is a characteristic view of a sensitivity distribution, wherein the horizontal axis represents axial positions of the photoconductor and the vertical axis represents sensitivity. Reference symbol FH designates a direction in which sensitivity is high, and FL designates a direction in which sensitivity is low.
Reference symbol FX shown in FIG. 20(b) is an approximate straight line in the sensitivity distribution. Specifically, provided that a change in the sensitivity of the photoconductor 5 comes to approximate a straight line, sensitivity is understood to have a tendency of to become higher (or lower) from one axial end to the other axial end. In the example shown in FIG. 20(b), sensitivity is low at one end Aa within an image formation range A, and becomes higher at the other end Ab. The sensitivity distribution of the photoconductor affects inplane evenness of an image printed by a printer.
An optical scanning device having a rotary polygon mirror as an exposure device is used in the image forming apparatus of this type. JPA′214 describes an image forming apparatus equipped with such an optical scanning device.
As mentioned above, in an image forming apparatus using such a photoconductor, the axial film thickness of the photosensitive layer is changed by factors arising during processes for manufacturing the photoconductor, which in turn raises a problem of the distribution of sensitivity becoming uneven because of variations in the axial film thickness of a photosensitive layer. Therefore, the inplane unevenness in an image printed by a printer is impaired as a result of the distribution of sensitivity being made uneven, thereby raising a problem of occurrence of inconsistencies in the density of a printed image. However, the image forming apparatus described in JPA′214 raises a problem of a failure to address deterioration of image quality attributable to changes in the axial film thickness of the photosensitive layer of the photoconductor.
Furthermore, in the optical scanning device, dust or dirt in the air collides against and adheres to the reflection faces of the rotary polygon mirror with lapse of an operating time, thereby inducing stains. FIG. 21 is a descriptive view for describing a circumstance where the rotary polygon mirror 1 is stained.
As shown in FIG. 21, when a rotary polygon mirror 1 rotates in a direction designated by arrow Rb, the air existing around the polygon mirror flows in relation to respective reflection faces 2 in a direction opposite the rotating direction of the reflection faces, to thus generate air currents 4. The air currents 4 induce air turbulence at a position downstream of a boundary edge section 3 between adjacent reflection faces 2, 2. As a result of occurrence of such air turbulence, the dust or dirt particles contained in the air currents 4 become entangled in the air turbulence and collide against the respective reflection faces 2. Therefore, in each reflection face 2 of the rotary polygon mirror 1, an area 2′ located downstream of and close to the boundary edge section 3 (i.e., a front end of the reflection face 2 in the rotating direction thereof) is chiefly, noticeably stained. Staining of the area 2′ located downstream of and close to the boundary edge 3 signifies that an area located upstream of a deflected light beam; i.e., staining occurs in an area located close to a start point from which writing is started by the light beam (hereinafter called a “write start point”) in the main scanning direction. During the course of continued use of the laser printer, optical power of the light beam spot acquired at the write start point is decreased.
FIG. 22 is a characteristic view showing characteristics of a decrease in the quantity of reflected light induced by stains on the reflection faces of the rotary polygon mirror. The horizontal axis in FIG. 22 represents positions on the reflection face, where a write start position is designated by 0 and a write end position is designated by 1. Further, the vertical axis represents a relative quantity of reflected light, where 1 designates a normal value. As shown in FIG. 22, the quantity of light reflected from the respective stained reflection faces 2 of the rotary polygon mirror 1 that has operated for a given period of time is understood to decrease dependent on positions on the reflection faces of the rotary polygon mirror. Measurement was performed at the same incident angle.
FIG. 23 is a descriptive view showing an example of changes in the distribution of light quantity caused by a scanning direction of a light beam; i.e., a rotating direction of the polygon mirror, and time-varying stains on the deflective reflection faces. FIG. 23(a) is a characteristic chart showing the distribution of light quantity, wherein the horizontal axis represents positions of the photoconductor; reference symbol A designates a range where an image is formed; Ax designates a write start side; and Ay designates a write end side. The vertical axis represents the distribution of light quantity; reference symbol VH designates a direction in which the quantity of light is high; and VL designates a direction in which the quantity of light is low. Reference symbol Va designates a characteristic of the distribution of initial light quantity, and Vb designates a characteristic of the distribution of light quantity achieved after lapse of time. The rotary polygon mirror 1 rotates in the direction of arrow Ra, and a scanning optical system 13 causes the light beam to scan in the direction of arrow D. An image is formed within the range A on a scan face 14 of the photoconductor, and the distribution of light quantity decreases with lapse of time. The drop in the distribution of light quantity achieved at the write start side Ax is greater than that achieved at the write end side Ay.
As mentioned above, when the rotary polygon mirror is continuously used, stains arise on the reflection faces of the rotary polygon mirror. Reflectivity is decreased by the stains, and the optical power of the optical beam spot on the scan face is decreased. The stains on the reflection faces become uneven within the respective reflection faces, and hence the distribution of optical power in the main scanning direction on the scan face also becomes uneven.
When an image forming apparatus using such an optical scanning device; for instance, an electrophotographic laser printer, is continuously used, there arises a problem of inconsistencies arising in the density or color of an image for reasons of unevenness in the distribution of light power. Specifically, the image forming apparatus using a rotary polygon mirror encounters a problem which is caused by optical changes or time-varying changes in the intensity of light in the main scanning direction. However, the image forming apparatus defined in JPA′214 encounters a problem of a failure to address deterioration of image quality, which is caused by optical changes or time-varying changes in the intensity of light in the main scanning direction.